1. Field of the Invention
The present invention relates to a method for fabricating a microstructure array, a method for fabricating a mold or a master of a mold (in the specification the term xe2x80x9cmoldxe2x80x9d is chiefly used in a broad sense including both a mold and a master of a mold) for forming a microstructure array, a method for fabricating a microstructure array using the mold, and a microstructure array. This invention particularly relates to a mold for forming a microlens array, a method for fabricating the mold, and a method for fabricating the microlens array using the mold.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 or 3 microns to about 200 or 300 microns and an approximately semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, optical receivers and inter-fiber connections in optical communcation systems.
Meanwhile, earnest developments have been made with respect to a surface emitting laser and the like which can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been repeatedly decreased in size as semiconductor processing techniques have been developed and advanced. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing.
In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in the fields of optical information processing, such as optical parallel processing-operations, and optical interconnections. Furthermore, active or self-radiating type display devices, such as electroluminescence (EL) panels, have been enthusiastically studied and developed, and a highly-definite and highly-luminous display has been proposed. In such a display, there is a heightened desire for a microlens array which can be produced at a relatively low cost and with a large area, as well as with a small lens size and a large NA.
There are presently a number of prior art methods for fabricating microlenses.
In a prior art microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1) L51-54, 1981), a refractive index is raised at plural places in a substrate of multi-component glass. A plurality of lenses are thus formed at the places with a high-refractive index. In this method, however, the lens diameter cannot be large, compared with the intervals between lenses. Hence, it is difficult to design a lens with a large NA. Further, the fabrication of a large-area microlens array is not easy since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array. Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens quality, such as a focal length, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions. Furthermore, since a thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a misfit between their thermal expansion coefficients as an integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, a difficulty in joining or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another prior art microlens-array fabrication method using a resist reflow (or melting) method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p23-34, 1991), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warp and so forth, in contrast with the ion exchange method.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of resin, wetting conditions between the substrate and resin, and the heating temperature. Therefore, variations between lots are likely to occur while fabrication reproducibility per a single substrate surface is high.
Further, when adjacent lenses are brought into contact with each other due to the reflow, a desired lens profile cannot be secured due to the surface tension. Accordingly, it is difficult to achieve a high light-condensing efficiency by bringing the adjacent lenses into contact and decreasing an unused area between the lenses. Furthermore, when a lens diameter from about 20 or 30 microns to about 200 or 300 microns is desired, the thickness of deposited resin must be large enough to obtain a spherical surface by the reflow. It is, however, difficult to uniformly and thickly deposit the resin material having desired optical characteristics (such as refractive index and optical transmissivity). Thus, it is difficult to produce a microlens with a large curvature and a relatively large diameter.
In another prior art method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1 (1989)-261601), or a wet etching method (see Japanese Patent Application Laid-Open No. 5 (1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warp due to the difference in the thermal expansion coefficient can be solved, in contrast to the ion exchange method.
In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large investment in equipment is needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm2 (100 cm-square) because the electron beam impact area is limited.
Further, in the wet etching method, since an isotropic etching using a chemical action is principally employed, an etching of the metal plate into a desired profile cannot be achieved if the composition and crystalline structure of the metal plate vary even slightly. In addition, etching will continue unless the plate is washed immediately after a desired shape is obtained. When a minute microlens is to be formed, a deviation of the shape from the desired one is possible due to etching lasting during a period from the time a desired profile is reached to the time the microlens is reached.
Further, there also exists a mold fabrication method using an electroplating technique (see Japanese Patent Application Laid-Open No. 6 (1994)-27302). In this method, as illustrated in FIGS. 1A and 1B, an insulating film 103 having a conductive layer 101 formed on one surface thereof and an opening 104 is used, the electroplating is performed with the conductive layer 101 acting as a cathode, and a protruding portion or plated layer 105 acting as a mother mold for a lens is formed on a surface of the insulating film 103. A resist layer 110 is formed on the other surface of the conductive layer 101 to prevent the formation of a plated layer on this surface. The process of fabricating the mold by this method is simple, and cost is reduced.
In the method of FIGS. 1A and 1B, the diameter of the opening 104 needs to be less than 20 or 30 microns when a minute microlens of about 20 or 30 microns in diameter is required to be fabricated. In such a case, since a contact area between the plated layer 105 and the conductive layer 101 is small, there is a great fear that the protruding portion 105 falls due to a shearing stress occurring between those layers 101 and 105 when a lens or mold is formed by this structure. An anchor portion is provided in a bottom portion of the plated layer 105 to prevent that falling, but this is not enough to solve the problem.
There exists another mold fabrication method using the electroplating technique (see Japanese Patent Publication No. 64 (1989)-10169). In this method, as illustrated in FIGS. 2A to 2D, after a convex plated layer 205 is formed, a photoresist insulating layer 203 is removed, except its portion between the plated layer 205 and an electrode layer 201, and another plated layer 206 is thickly formed on the plated layer 205 and the electrode layer 201 to form a mold. There is, however, a fear that the mold deforms or cracks occur in the mold when heating of transparent resin to be molded and pressure molding are repeatedly conducted using the thus-fabricated mold. Those phenomena are due to the fact that thermal and mechanical strains tend to be accumulated since mechanical characteristics, such as Young""s modulus and yielding strength, of the photoresist 203 left between the plated layer 205 and the electrode layer 201 are far smaller than those of the other elements, and the fact that the molecular weight of high polymer resin, such as the photoresist, tends to be lowered and hence the resin is gasified.
An object of the present invention is to provide a fabrication method for fabricating a microstructure array (typically a microlens array such as a semispherical microlens array, a flyeye lens and a lenticular lens) with a high resistivity flexibly, readily and stably, a fabrication method of a mold for forming a microstructure array, a fabrication method of a microstructure array using the mold, and so forth.
The present invention is generally directed to a fabrication method for fabricating an array of microstructures which includes the following steps:
preparing a substrate with an electrically-conductive portion;
forming an insulating mask layer on the electrically-conductive portion;
forming a plurality of openings in the insulating mask layer to expose the electrically-conductive portion;
forming a first plated or electrodeposited layer in the opening and on the insulating mask layer by electro- or electroless-plating or electrodeposition, with at least a surface of the first plated or electrodeposited layer being electrically conductive;
removing the insulating mask layer; and
forming a second plated layer on the first plated or electrodeposited layer and on the electrically-conductive portion by electroplating.
More specifically, the following constructions are possible based on the above fundamental construction.
The step of forming the first plated or electrodeposited layer may be stopped after a thickness or height of the first plated or electrodeposited layer (i.e., the distance between the exposed electrically-conductive portion and the top of the first plated layer) reaches half a designed final height or more of the second plated layer above a central portion of the opening.
When the electroplating is performed after the mask layer of insulating material such as resist is removed, the cathode is composed of the electrically-conductive portion and the first plated or electrodeposited layer. In such a case, a current tends to be concentrated on a top portion of the first layer, and hence, the semispherical or semicylindrical profile of the plated layer is likely to be deformed. Further, the height of the plated layer is likely to differ between a peripheral portion of its array and a central portion of its array. In contrast, when the electroplating is performed with the mask layer present, the cathode is composed of the plated or electrodeposited layer only. In such a state, an approximately uniform current density can be obtained over the semispherical or semicylindrical plated or electrodeposited layer. Therefore, it is preferable to form the plated or electrodeposited layer with a desired profile under the condition that the mask layer is present, as far as possible. For this purpose, the first-layer forming step is continued after its thickness reaches or exceeds half a designed final height of the second plated layer above the central portion of the opening (i.e., the distance between the exposed electrically-conductive portion and the top of the second plated layer). In this case, the height of the plated or electrodeposited layer is proportional with a curvature radius thereof.
The first plated or electrodeposited layers may be formed in the step of forming the first plated or electrodeposited layer such that the insulating mask layer between the adjacent first plated or electrodeposited layers is not completely covered.
The step of forming the second plated layer may be stopped after a thickness of the second plated layer (i.e., the distance between the top of the first plated layer and the top of the second plated layer) reaches a thickness of the insulating mask layer or more. This is needed to completely fill the space created by the removal of the mask layer with the second plated layer and to firmly fix the first layer to the substrate.
The opening may have a circular shape and the microstructure may be a semispherical microstructure, or an elongated, striped shape, and the microstructure may be a semicylindrical microstructure.
The fabrication method may further include a step of forming a mold on the the substrate with the first plated or electrodeposited layer and the second plated layer, and a step of separating the mold from the substrate. In this case, the mold may be formed using electroplating, and the mold may be a mold for fabricating a microlens array.
In this method, the mold can be directly formed by electroplating or the like. Therefore, no expensive equipment is needed, costs can be reduced, and the size of the mold can be enlarged readily. Furthermore, the size of the plated layer can be controlled in situ, and the lens diameter and the like can be readily and precisely controlled by controlling electroplating time and temperature.
The fabrication method may further include a step of coating a light-transmitting material on the mold, a step of hardening the light-transmitting material, and a step of separating the material from the mold to obtain the microlens array.
The present invention is also directed to a microstructure array including:
a substrate having an electrically-conductive portion;
a first plated or electrodeposited layer formed on the electrically-conductive portion by electro- or electroless-plating or electrodeposition using a plurality of openings formed in an insulating mask layer formed on the electrically-conductive portion of the substrate to expose the electrically-conductive portion, at least a surface of the first plated or electrodeposited layer being electrically conductive, and the insulating mask layer being finally removed; and
a second plated layer formed on the first plated or electrodeposited layer and on the electrically-conductive portion by electroplating.
More specifically, the following structures are possible based on the above fundamental structure.
A height of the second plated layer formed on the first plated or electrodeposited layer above a central portion of the opening (i.e., the distance between the exposed electrically-conductive portion and the top of the second plated layer) may be in a range from 1 xcexcm to 100 xcexcm.
A thickness of the second plated layer formed on the first plated or electrodeposited layer above a central portion of the opening (i.e., the distance between the top of the first plated layer and the top of the second plated layer) may be equal to or smaller than a thickness or height of the first plated or electrodeposited layer above the central portion of the opening (i.e., the distance between the exposed electrically-conductive portion and the top of the first plated layer).
The first plated or electrodeposited layers may be formed so as not to completely cover the electrically-conductive portion.
A thickness of the second plated layer may be equal to or larger than a spacing between the first plated layer and the electrically-conductive portion.
These advantages and others will be more readily understood in connection with the following detailed description of the more preferred embodiments in conjunction with the drawings.